Computer Networks Principles Network Layer - IP

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1 Computer Networks Principles Network Layer - IP Prof. Andrzej Duda duda@imag.fr 1

2 Network Layer Overview: Datagram service IP addresses Packet forwarding principles Details of IP 2

3 Datagram networks: the Internet model no call setup at network layer routers: no state about end-to-end connections no network-level concept of connection packets typically routed using destination host ID packets between same source-dest pair may take different paths application transport network data link physical 1. Send data 2. Receive data application transport network data link physical 3

4 IP principles Elements host = end system; router = intermediate system; subnetwork = a collection of hosts that can communicate directly without routers Routers are between subnetworks only: a subnetwork = a collection of systems with a common prefix Packet forwarding direct: inside a subnetwork hosts communicate directly without routers, router delivers packets to hosts indirect: between subnetworks one or several routers are used Host either sends a packet to the destination using its LAN, or it passes it to the router for forwarding 4

5 Interconnection structure - layer 3 interconnection layer 3 router subnetwork 1 subnet 3 subnet 2 switch (bridge) VLAN host 5

6 Interconnection at layer 3 Routers interconnect subnetworks logically separate groups of hosts managed by one entity Forwarding based on IP address structured address space routing tables: aggregation of entries works if no loops - routing protocols scalable inside one administrative domain 6

7 Internet and intranet An intranet a collection of end and intermediate systems interconnected using the TCP/IP architecture normally inside one organization The Internet the global collection of all hosts and routers interconnected using the TCP/IP architecture coordinated allocation of addresses and implementation requirements by the Internet Society Intranets are often connected to the Internet by firewalls routers that act as protocol gateways (address and port translation, application level relay) 7

8 IP addresses Unique addresses in the world, decentralized allocation An IP address is 32 bits, noted in dotted decimal notation: An IP address has a prefix and a host part: prefix:host Two ways of specifying prefix subnet mask identifies the prefix by bitwise & operation CIDR: bit length of the prefix Prefix identifies a subnetwork used for locating a subnetwork - routing 8

9 IP Addressing: introduction IP address: 32-bit identifier for host, router interface interface: connection between host, router and physical link router s typically have multiple interfaces host may have multiple interfaces IP addresses associated with interface, not host, router interface interface3 interface bits =

10 IP Addressing IP address: network (or prefix) part (high order bits) host part (low order bits) What s a subnetwork? (from IP address perspective) device interfaces with same network part of IP address can physically reach each other without intervening router Network LAN network consisting of 3 IP networks (for IP addresses starting with 128, first 24 bits are network address) 10

11 ezci7-ethz-switch ETHZ-Backbone Example Komsys ezci7-ethz-switch Modem + PPP sic500cs ed0-ext EPFL-Backbone ed0-swi stisun in-inr lrcsuns 00:00:0C:02:78: LRC 08:00:20:71:0D:D4 lrcpc :00:C0:B8:C2:8D ed2-in DI lrcmac :00:07:01:a2:a Switch x.x in-inj :00:0C:17:32:96 disun :00:20:20:46:2E Ring SIDI SUN ed2-el LEMA lrcmac :00:07:01:a2:a5 11

12 IP Address Classes class A class B class C class D class E Net Id Subnet Id Host Id 10 Net Id Subnet Id Host Id 110 Net Id Host Id 1110 Multicast address Reserved Examples: Class A B C D E x.x = EPFL host; x.x = ETHZ host 9.x.x.x = IBM host 18.x.x.x = MIT host Range to to to to to Class B addresses are close to exhausted; new addresses are taken from class C, allocated as continuous blocks 12

13 Special case IP addresses this host, on this network 2. 0.hostId specified host on this net (initialization phase) limited broadcast (not forwarded by routers) 4. subnetid.all 1 s broadcast on this subnet 5. subnetid.all 0 s BSD used it for broadcast on this subnet (obsolate) x.x.x loopback 7. 10/8 reserved networks for /12 internal use (Intranet) /16 n 1,2: source IP@ only; 3,4,5: destination IP@ only 13

14 Used addresses in Internet 14

15 CIDR: IP Address Hierarchies The prefix of an IP address is itself structured in order to support aggregation For example: x.y represents an EPFL host / 24 represents the LRC subnet at EPFL /15 represents EPFL Used between routers by routing algorithms This way of doing is called classless and was first introduced in inter domain routing under the name of CIDR (Classless Interdomain Routing) Notation: /16 means : the prefix made of the 16 first bits of the string It is equivalent to: with netmask= In the past, the class based addresses, with networks of class A, B or C was used; now only the distinction between class D and non-class D is relevant. 15

16 CIDR addresses /21: C class network: 256 addresses = 2048 addresses 16

17 Choosing prefix length addresses 29 3 prefix = /29 8 addresses 2 broadcast addresses: , only 6 addresses can be used for hosts 17

18 Choosing prefix length addresses prefix = / /28, /28, /28 16 addresses 2 broadcast addresses: , only 14 addresses can be used for hosts 18

19 Address allocation World coverage Europe and the Middle East (RIPE NCC) Africa (ARIN & RIPE NCC) North America (ARIN) Latin America including the Caribbean (ARIN) Asia-Pacific (APNIC) Current allocations of Class C /8, /8, 217/8 for RIPE /8, /8, 216/8 for ARIN /8, /8, 218/8 for APNIC Simplifies routing short prefix aggregates many subnetworks routing decision is taken based on the short prefix 19

20 Number of hosts 20

21 21

22 IP Addresses and subnet mask subnet mask at ETHZ = CIDR /16 subnet mask at KTK = CIDR /26 question: subnet prefix and host parts of spr13.tik.ee.ethz.ch = ? : : answer: subnet prefix = (64= ) host = 13= (6 bits) 22

23 IP Addresses and subnet mask 23

24 IP Addresses /24 Java Business Solutions AG /23 Tango SA /16 Internet Service Provider SovKom Sovkom has received IP addresses to total: 2 16 addr., but.0 and.255 are not usable Java Business Solutions AG has received IP addresses to Tango SA total: addresses has received IP addresses to total: addresses 24

25 Example switch? ?? ? ? switch host A n Can host A have this address? 25

26 Example switch host X host Y router host Z router switch host A n Host A is on subnetwork

27 IP Principles Homogeneous addressing an IP address is unique across the whole network ( = the world in general) IP address is the address of the interface communication between IP hosts requires knowledge of IP addresses Routing: inside a subnetwork: hosts communicate directly without routers between subnetworks: one or several routers are used a subnetwork = a collection of systems with a common prefix 27

28 IP packet forwarding algorithm Rule for sending packets (hosts, routers) if the destination IP address has the same prefix as one of my interfaces, send directly to that interface otherwise send to a router as given by the IP routing table At lrcsuns: Next Hop Table Physical Interface Tables destination@ subnetmask nexthop IP subnetmask DEFAULT At in-inj: Next Hop Table Physical Interface Tables destination@ subnetmask nexthop IP subnetmask DEFAULT

29 ezci7-ethz-switch ETHZ-Backbone Example Komsys ezci7-ethz-switch Modem + PPP sic500cs ed0-ext EPFL-Backbone ed0-swi stisun in-inr lrcsuns 00:00:0C:02:78: LRC 08:00:20:71:0D:D4 lrcpc :00:C0:B8:C2:8D ed2-in DI lrcmac :00:07:01:a2:a Switch x.x in-inj :00:0C:17:32:96 disun :00:20:20:46:2E Ring SIDI SUN ed2-el LEMA lrcmac :00:07:01:a2:a5 29

30 IP packet forwarding algorithm destaddr = packet dest. address, destinationaddr = address in routing table Case 1: a host route exists for destaddr for every entry in routing table if (destinationaddr = destaddr) then send to nexthop IPaddr; leave Case 2: destaddr is on a directly connected network (= on-link): for every physical interface IP address A and subnet mask SM if(a & SM = destaddr & SM) then send directly to destaddr; leave Case 3: a network route exists for destaddr for every entry in routing table and subnet mask SM if (destinationaddr & SM = destaddr & SM) then send to nexthop IP addr; leave Case 4: use default route for every entry in routing table if (destinationaddr=default) then send to nexthop IPaddr; leave 30

31 Getting a datagram from source to dest. IP datagram: misc fields source IP addr dest IP addr data datagram remains unchanged, as it travels source to destination addr fields of interest here A B routing table in A Dest. Net. next router Nhops default to Internet E

32 Getting a datagram from source to dest.: same subnetwork misc fields data Starting at A, given IP datagram addressed to B: look up net. address of B find B is on same net. as A link layer will send datagram directly to B inside link-layer frame B and A are directly connected P P A B Dest. Net. next router P P Nhops default to Internet E 32

33 Getting a datagram from source to dest.: different subnetworks misc fields data Starting at A, dest. E: look up network address of E E on different network A, E not directly attached routing table: next hop router to E is link layer sends datagram to router inside linklayer frame datagram arrives at continued.. P A B Dest. Net. next router to Internet P P Nhops default E 33

34 Getting a datagram from source to dest.: different subnetworks misc fields data Arriving at , destined for look up network address of E E on same network as router s interface router, E directly attached link layer sends datagram to inside link-layer frame via interface datagram arrives at !!! (hooray!) default xx xx A B Dest. next network router Nhops interface P P P E P 34

35 IP datagram format IP protocol version number header length (bytes) type of data max number remaining hops (decremented at each router) upper layer protocol to deliver payload to ver head. len 16-bit identifier time to live 32 bits type of service upper layer flgs length data (variable length, typically a TCP or UDP segment) fragment offset Internet checksum 32 bit source IP address 32 bit destination IP address Options (if any) total datagram length (bytes) for fragmentation/ reassembly E.g. timestamp, record route taken, specify list of routers to visit. 35

36 IP header Version IPv4, futur IPv6 Header size options - variable size in 32 bit words Type of service priority : 0 - normal, 7 - control packets short delay (telnet), high throughput (ftp), high reliability (SNMP), low cost (NNTP) Redefined in DiffServ (Differentiated Services) 1 byte codepoint determining QoS class Expedited Forwarding (EF) - minimize delay and jitter Assured Forwarding (AF) - four classes and three drop-precedences (12 codepoints) 36

37 IP header Packet size Id Flags in bytes including header in bytes including header <= 64 Kbytes; limited in practice by link-level MTU (Maximum Transmission Unit) every subnet should forward packets of 576 = bytes unique identifier for re-assembling M : more ; set in fragments F : prohibits fragmentation 37

38 IP header Offset position of a fragment in multiples of 8 bytes TTL (Time-to-live) in secondes now: number of hops router : --, if 0, drop (send ICMP packet to source) Protocol identifier of protocol (1 - ICMP, 6 - TCP, 17 - UDP) Checksum only on the header 38

39 IP header Options strict source routing all routers loose source routing some routers record route timestamp route router alert used by IGMP or RSVP for processing a packet 39

40 LAN Addresses and ARP 32-bit IP address: network-layer address used to get datagram to destination network (recall IP network definition) LAN (or MAC or physical) address: used to get datagram from one interface to another physicallyconnected interface (same network) 48 bit MAC address (for most LANs) burned in the adapter ROM Why different addresses at IP and MAC? LANs not only for IP (LAN addresses are neutral) if IP addresses used, they should be stored in a RAM and reconfigured when host moves independency of layers 40

41 MAC Address resolution Starting at A, given IP datagram addressed to B: look up net. address of B, find B on same net. as A link layer send datagram to B inside link-layer frame frame source, dest address A B datagram source, dest address E B s MAC addr A s MAC addr B s IP addr A s IP addr IP payload frame datagram 41

42 Example bridge host X host Y router host Z router bridge host A n Host A is on subnetwork

43 Packet delivery Packet sent by to MAC-host-Y MAC-host-X payload Ethernet header IP header X needs to know MAC address of Y (ARP) Packet sent by to MAC-router MAC-host-X payload Ethernet header IP header MAC-host-A MAC-router payload Ethernet header IP header X needs to know MAC address of router (X knows the IP address of router - configuration) Router needs to know MAC address of A 43

ARP: Address Resolution Protocol ARP is used to determine the MAC address of B given B s IP address Each IP node (Host, Router) on LAN implements ARP protocol and has ARP table ARP

44 ARP: Address Resolution Protocol ARP is used to determine the MAC address of B given B s IP address Each IP node (Host, Router) on LAN implements ARP protocol and has ARP table ARP Table: IP/MAC address mappings for some LAN nodes < IP address; MAC address> <.. > ARP table is a cache: after an interval (typically 20 min) the address mapping will be forgotten 44

45 ARP protocol A knows B's IP address, wants to learn physical address of B A broadcasts ARP query pkt, containing B's IP address all machines on LAN receive ARP query B receives ARP packet, replies to A with its (B's) physical layer address A caches (saves) IP-to-physical address pairs until information becomes old (times out) soft state: information that times out (goes away) unless refreshed 45

ARP protocol IP address MAC address TTL 10.

46 ARP protocol IP address MAC address TTL :BD:D2:07:56:2A 6:00:00 A: IP dest IP source MAC source ARP packet IP dest IP source MAC source MAC dest IP dest = my IP addr B:

47 ARP frame Request (broadcast) Reply (unicast) sender Ethernet address sender IP address target Ethernet address??? target IP address sender Ethernet address sender IP address target Ethernet address target IP address 47

48 Routing to another LAN walkthrough: routing from A to B via R A R B In routing table at source Host, find router In ARP table at source, find MAC address E6-E BB-4B, etc 48

49 Proxy ARP Proxy ARP: a host answers ARP requests on behalf of others example: sic500cs for PPP connected computers manual configuration of sic500cs Modem + PPP sic500cs stisun ed0-ext EPFL-Backbone ed2-in 49

50 ICMP: Internet Control Message Protocol Used by hosts, routers, gateways to communication network-level information error reporting: unreachable host, network, port, protocol echo request/reply (used by ping) Network-layer above IP: ICMP msgs carried in IP datagrams ICMP message: type, code plus first 8 bytes of IP datagram causing error Type Code description 0 0 echo reply (ping) 3 0 dest. network unreachable 3 1 dest host unreachable 3 2 dest protocol unreachable 3 3 dest port unreachable 3 6 dest network unknown 3 7 dest host unknown 4 0 source quench (congestion control - not used) 8 0 echo request (ping) 9 0 router advertisement 10 0 router discovery 11 0 TTL expired 12 0 bad IP header 50

ICMP Redirect Sent by router to source host to inform source that destination is directly

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Type=5 code checksum

51 ICMP Redirect Sent by router to source host to inform source that destination is directly connected host updates the routing table ICMP redirect can be used to update the router table (eg. in-inj route to LRC?) ICMP Redirect Format / / IP datagram header (prot = ICMP) Type=5 code checksum Router IP address that should be preferred IP header plus 8 bytes of original datagram data / / 51

52 ICMP Redirect example in-inr lrcsuns ed2-in inr-el ed2-el lemas dest IP addr srce IP addr prot data part 1: udp xxxxxxx 2: udp xxxxxxx 3: icmp type=redir code=host cksum xxxxxxx (28 bytes of 1) 4: udp... 52

53 ICMP Redirect example (cont d) After 4 lrcsuns$ netstat -nr Routing Table: Destination Gateway Flags Ref Use Interface UH lo UGHD U le U 3 0 le0 default UG

54 Tools that use ICMP ping ICMP Echo request wait for Echo reply measure RTT traceroute IP packet with TTL = 1 wait for ICMP TTL expired IP packet with TTL = 2 wait for ICMP TTL expired... 54

55 Traceroute source router A router B dest TTL=1 t A TTL=2 t B 55

56 Summary The network layer transports packets from a sending host to the receiver host. Internet network layer connectionless best-effort Main components: addressing packet forwarding routing protocols and routers (or how a router works) Routing protocols will be seen later 56

Computer Networking. Network Layer - IP. Prof. Andrzej Duda

Computer Networking. Network Layer - IP. Prof. Andrzej Duda Computer Networking Network Layer - IP Prof. Andrzej Duda duda@imag.fr http://duda.imag.fr 1 The transport layer relies on the services of the network layer, which provides a communication service between